Results following administration of BCG to tumors in animal models prompted the study of the influence of BCG vaccination on leukemia mortality in humans. Davignon, in Quebec, in 1970, found half as high a rate (per 100,000) of leukemia mortality among vaccinated children compared with that for the balance of children of similar ages. This degree of diminution has not been duplicated by others, either in the U.S., or elsewhere. In 1971, Comstock investigated the incidence of leukemia, lymphosarcoma, and Hodgkin's disease in Muscogee County, Georgia where a community-wide controlled trial of BCG took place, in 1950. He found a case rate of 82.8 (per 100,000) for vaccines to be slightly lower than that of 89.6, among controls (not statistically significant). In 1972, he investigated cancer mortality and morbidity in Puerto Rico, where a controlled trial of BCG took place in 1949-51, among school children. Perhaps the most important finding was excess lymphosarcoma and Hodgkin's disease (17 cases) among vaccinees compared with 2 cases which would have been expected, had the rate among controls prevailed. Slightly more cancer deaths were observed among vaccinees (40) than expected (24), for all forms; there were also more cases observed among vaccinees (98) than expected (69). The average annual case rate for all forms was 10.3 among vaccinees compared with 7.2 among controls, though the difference is not statistically significant at the 5% level. Among 14- and 15-year olds in England, the British Medical Research Council found a rate for lymphatic and hematopoietic tissues lower among vaccinees (2.9 per 100,000) than among controls (4.1); the mortality rate for other malignancies was higher among vaccinees (3.4) than that for controls (2.1), (neither difference being statistically significant). These findings make it highly desirable to study the incidence of all forms of cancer in the only community-wide controlled trial of BCG in the United States.